The present invention relates to imaging systems and, more particularly to correction of images for systematic brightness variations across the image.
Although the present invention may be employed for correction of images created by any process, and the scope of the invention should be so construed, for concreteness of description, the present invention is described in connection with the correction of images produced by nuclear magnetic resonance devices.
A nuclear magnetic resonance device produces an image of an object by subjecting the object to a constant magnetic field having one or more magnetic gradients superimposed thereon and exciting nuclei in the object with a properly chosen band of radio frequencies. The magnetic moments of the nuclei of, for example, hydrogen, are rotated into a transverse plane by a predetermined radio frequency pulse length. Following the end of the radio frequency pulse, the magnetic moments return to alignment with the magnetic field. As they do so, they emit small signals at the same radio frequencies as those in the excitation pulse. The emitted radio frequencies are detected to sense the density of hydrogen nuclei within a thin slice of the object being imaged. A complete disclosure of the theory and practice of nuclear magnetic resonance imaging is contained in U.S. Pat. Nos. 4,431,968 and 4,444,760, commonly assigned with the present invention, the disclosures of which are herein incorporated by reference as background material.
Hydrogen nuclii are conveniently available in large quantities in many soft tissues of interest, and the images produced therefrom are of significant value. Proper selection of magnetic gradients and radio frequencies enable imaging based on magnetic resonance of other materials such as, for example, sodium.
The radio signals emitted by atomic nuclei are very weak. It is conventional to employ surface coils placed in close proximity to the surface of the object being imaged to improve the signal to noise ratio of the received radio signals. Such surface coils may be used only for reception in cooperation with transmitting coils spaced from the object or they may be employed both for transmitting and receiving the radio frequency signals.
The images produced with surface coils sometimes contain artifacts of the imaging technique complicating image interpretation. One problem is systematic attenuation of the radio frequencies as they traverse the object being imaged. If the thin slice being imaged is transverse to, for example, a human torso, attenuation of the radio frequency signal increases for deeper parts of the torso making the received signal strengths from parts near the surface substantially greater than those at greater depths within the torso. As a result, parts near the surface show up substantially brighter on the received image than those located deeper within the object. Such range-determined image variation is a product of the imaging system and not of the anatomy being imaged.
Another problem arises from the brighter image and increased attenuation produced by some types of tissue such as, for example, fatty tissue. If a mass of fatty tissue is disposed close to the surface in the slice being imaged, the brightness of its image may exceed the dynamic range of the imaging device such as, for example, a cathode ray tube. One solution includes adjusting the image brightness to retain the brightest area within the dynamic range of the imaging device. In many cases, the objects of interest within the slice is located outside the brightly imaged fatty area. If the overall image brightness is reduced to retain the brightness of the fatty area within the dynamic range of the display, then the less bright areas of interest are even further dimmed to the point that the desired information may not be discernable.
Areas of excessive brightness have a further disadvantage. One attempting to analyze the less bright, but information-containing parts of an image may experience eyestrain and discomfort by the nearby presence of the excessively bright area in the image.
The greater attenuation of the near-surface fatty area also may reduce the received radio signals from deeper body elements positioned such that the radio signals must pass through the fatty area. Thus an additional systematic range-dependent reduction in image brightness is encountered.